CN1886805B - Production method of nuclear fuel pellets - Google Patents

Abstract

The nuclear fuel pellets are produced by sintering a material containing U02 uranium dioxide obtained from a powder from a UF6 uranium hexafluoride conversion process. The powder obtained directly from a UF6 uranium hexafluoride conversion process is introduced into a vessel containing moving compression and mixing bodies and the vessel is agitated in such a way that the powder is displaced inside the volume of the vessel according to three non-coplanar axes such that it is compressed between the moveable bodies and between the moveable bodies and the walls of the vessel until a particulate material is formed, having a higher density in relation to the powder obtained by the conversion process and the particulate material directly obtained by agitation inside the vessel is used to shape the raw fuel pellets which undergo sintering. Preferably, the vessel is set into a vibratory motion during treatment. Extremly varied additives can be introduced into the vessel either before or duringtreatment, by agitation inside the vessel.

Description

Production method of nuclear fuel pellets

The invention relates to a method for producing nuclear fuel pellets mainly comprising uranium dioxide _UO2 , which is used for the manufacture of fuel elements for nuclear reactors.

Fuel elements for nuclear reactors, and especially for pressurized water-cooled nuclear reactors, generally consist of long closed-ended tubes filled with fuel pellets, usually slightly less than 10 mm in diameter and 10 mm to 20 mm in length.

The fuel pellets are obtained by sintering a material mainly comprising uranium dioxide UO ₂ , obtained from the powder produced by the conversion process of uranium hexafluoride UF ₆ , at _temperatures generally close to 1700° C.

Different processes are known for obtaining uranium oxides, in particular uranium dioxide, by transformation from uranium hexafluoride. In particular, the so-called "dry conversion process" is known for the conversion of gaseous UF6, in which UF6 is pyrohydrolyzed by means of steam. Oxides whose average composition can be represented by the molecular formula UO _2+x can be obtained by this method. These oxides mainly consist of the dioxide UO ₂ and other oxides such as U ₃ O ₈ or U ₃ O ₇ in various proportions, depending on how the UF ₆ conversion process is carried out, the powder obtained by the dry process is containing very small Low-density powder (density typically less than 1 g/cm ³ ) of small-scale (0.1 μm to 0.4 μm) crystallites that are agglomerated to a greater or lesser degree. This powder has fair flowability (measured by standard flow test).

When producing fuel pellets, raw material pellets must be produced by cold pressing the granular material prior to sintering. In the case of industrial manufacturing, the manufacture of raw pellets by pressing requires high rates of placement of the granular material into deep and narrow cylindrical molds, so the granular material used to make the raw pellets that are subsequently sintered must have good flow properties and from this material it is possible to obtain feedstock pellets which are sufficiently strong to withstand handling before sintering.

Various methods of improving the mechanical properties _of raw pellets are known (for example the addition of a defined mass of _U3O8 powder as described in French Patent 2,599,883 and European Patent 0249,549). These methods are generally based on the addition _of controlled amounts _of oxides, such as _U3O8 or _U3O7 , to _UO2 . Typically, additives, such as lubricants and porogens, that aid in the shape of the feed pellets and can control the porosity and density of the sintered fuel pellets must also be combined with the particulate material from which the feed pellets are made.

The uranium oxide powder obtained by the UF6 conversion process, especially the dry process, cannot be used untreated to make raw material pellets. A number of operations are generally necessary in order to obtain a granular material with good flow properties, which is significantly denser than the powder density and has the desired properties necessary to obtain good quality raw material pellets. In particular, the particle size of the powder must be increased and made consistent in order to obtain particles of sufficient size and shape for improved flowability and compressibility.

Usually, the powder obtained directly from the dry conversion process is first sieved in the conversion plant and/or the hard particles (such as fluorinated particles) remaining during the sieving are ground, the powder is homogenized, packed and stored for For a pellet production unit, which may or may not be located adjacent to the conversion unit.

The powder is then charged to a pelletizing unit mixer and additives, in particular pore-forming additives, are added, the powder is then mixed and homogenized with the additives, and the powder mixture is pre-compacted in a press to obtain a pre-compacted material. The pre-compacted material is then subjected to a granulation operation in a mill or granulator, followed by a spheroidization operation in an agitated vessel in order to obtain regular shaped particles close to spherical. A lubricant is then added, mixed with the granules by agitation, before applying the compaction, in order to obtain raw material pellets which will subsequently be sintered.

Therefore the transformation of the powder obtained directly from the UF ₆ conversion process into a granular material which can be compressed into the form of raw pellets requires many operations, all of which must be carried out under satisfactory conditions so that good quality raw pellets and sintered pellets can be obtained . All these operations require many different pieces of equipment such as mixers, presses (roll presses) and pelletizing mills, which can fail for many reasons. The main risk point is the failure of the sieve at the exit of the granulator, which is required to ensure that the granulated material from which the feed pellets are produced has a good consistent particle size. If the screen fails, the resulting product needs to be reprocessed to remove the metal residue from the screen damage and to ensure the proper particle size required for the pellet production operation.

It is also known to produce uranium oxide powders by the wet process using suspension spraying. The powder obtained by the "wet" process has better density and fluidity than powder obtained by the dry process, and has particle size characteristics that cannot be directly obtained by currently known dry processes. However, these powders must also be conditioned before being formed into raw material pellets to be sintered. Furthermore, due to safety and environment-related factors, it is increasingly desirable to replace wet processes with dry processes, and it is increasingly necessary to provide dry-derived _UO2 powders for plants that typically utilize wet-process products.

In particular, when manufacturing mixed MOX fuels containing a mixture of uranium dioxide _UO2 and plutonium dioxide _PuO2 , it is necessary to provide the manufacturing unit with _UO2 powder obtained by the dry process.

The currently used manufacturing method of mixed pellets of uranium oxide and plutonium oxide requires the use of _UO2 powder with good flowability, which contains preferably regular-shaped particles, has a high density close to 2g/ ^cm3 , and is controlled at 250μm particle size below , in order to obtain a good mixture of uranium oxide and plutonium oxide, and to obtain the properties of raw material pellets with good mechanical strength.

In order to provide dry powders with good density, fluidity and particle size properties, a process of spray-drying uranium oxide powders obtained by industrially feasible dry processes has been proposed for oxidation containing limited concentrations of the 235U isotope Uranium powder.

It has also been proposed that, in order to improve the flow, density and particle size properties of dry powders, granules can be produced by pre-compacting followed by granulation. However, the particles obtained are too large (up to 1200 μm) to be intimately mixed with the plutonium oxide powder. Further granulation and grinding operations with particle sieving are therefore necessary. Conventional milling techniques can adversely affect flow properties and reduce product density. Furthermore, this operation is complicated and poses a certain risk that the sieve wires used may be damaged, so that debris may be mixed into the granules, which may cause damage to the pellet-forming equipment on which the granules are used. More commonly, in addition to the addition of plutonium oxide _PuO2 in the manufacture of MOX fuel, it may be necessary to add various additives to the dry-processed _UO2 powder. These additives may be moderator materials _such as absorbents and fissile fuel species such as _ThO2 or _rare earth metal oxides such as _Gd2O3 , _Er2O3 . In order to add these additives and mix them with the _UO2 powder obtained by the dry process, the _UO2 powder must be subjected to a previous conditioning treatment, for example by a greater or lesser degree of homogenization or spray drying, followed by pre-compression and granulation operations , these operations are followed by one or more stages of grinding and/or sieving. These processes are therefore complex and require multiple stages of conditioning and mixing the _UO2 powder with additives.

As mentioned above, in the manufacture of fuels mainly based on uranium oxide _UO2 , a mixture of _UO2 and _U3O8 (or _U3O7 ) is usually prepared in a ratio of 80/20 or preferably 90/10. The U3O8 oxide used can be directly obtained by using a dry process by adjusting the conditions for steam pyrolysis of _UF6 . U ₃ O ₈ or U ₃ O ₇ can also be obtained by low-temperature oxidation of UO ₂ powder. The oxides U ₃ O ₈ or U ₃ O ₇ can be added to the initial powder or to the mixture in the pellet unit before the pre-compression stage.

Other additives for adjusting the microstructure of fuel pellets such as chromium oxide, aluminum oxide, silicon oxide, vanadium oxide and niobium oxide or other compounds can be added at different stages in the process and mixed with dry obtained uranium dioxide, in all In this case, this requires the preparation of a particulate material with the properties required to make the feedstock pellets. These addition and mixing operations can further complicate the preparation of the particulate material.

The use of lubricants is often required at certain stages of the process, for example prior to pre-compaction and prior to compression of the particulate material into raw pellet form.

In particular, the operations that must be carried out prior to the manufacture of the raw material pellets are numerous and complex, especially in the case of the use of dry processes to obtain fine uranium oxide _UO2 , which has little or no fluidity, in which it is necessary to use to replace the wet process.

It is therefore an object of the present invention to provide a method for the manufacture of nuclear fuel pellets comprising the sintering of a material comprising uranium dioxide _UO2 obtained from a powder produced by the _UF6 conversion process, by which method it is possible to obtain simplified Manipulation of granular material of uranium dioxide _UO2 , which has suitable properties for the manufacture of raw material pellets for subsequent sintering.

For this purpose, the powder obtained directly from the conversion process of uranium hexafluoride UF ₆ is placed in a container containing a kinematic compaction and mixing body and the container is agitated in order to cause the powder to flow in the internal volume of the container along three non-coplanar Axis movement for compaction of the powder between the moving bodies and between the moving bodies and the walls of the container until a granular material with a density in the undensified state of at least 1.7 g/ ^cm3 is formed using this obtained by agitation inside the container The particulate material is densified to form raw fuel pellets to be sintered.

The method according to the invention may comprise the following features independently or in combination:

- The container performs a three-dimensional vibratory movement;

- the powder placed in the container is obtained by a dry conversion process and has a density of less than 1 g/cm ³ , and the density of the undensified granular material obtained by agitation in the container is about 2 g/cm ³ ;

- The powder obtained directly by the UF ₆ hexafluoride conversion process has a density of less than 1 g/cm ³ and zero flowability as determined by a standard test through a 15 mm orifice, and after agitation in the container for several minutes, in the container by agitation The obtained granular material has a fluidity greater than 10 g/s;

- Agitation of containers containing moving objects and powders directly obtained from UF ₆ hexafluoride conversion process for 1 to 600 minutes;

- kinematic compacted and mixed bodies in containers are free bodies with any simple geometry and low-roughness surfaces;

- the moving object is cylindrical;

- the moving object has the shape of a roughly spherical bead;

- moving bodies made of one of the following materials: sintered alumina Al ₂ O ₃ , sintered uranium oxide, sintered pure or doped zirconia, tungsten carbide, steel, uranium metal or uranium/titanium alloy;

- before agitating the vessel, at least one additive comprising at least one nucleating agent is added to the vessel in a proportion of at least 0.01% together with uranium dioxide _UO2 powder obtained directly from the _UF6 hexafluoride conversion process ;

- at least one additive is placed in the container together with uranium dioxide _UO2 powder obtained directly using the _UF6 hexafluoride conversion process;

- placing the additive in the container prior to agitating the container;

- the introduction of additives into the container during the agitation process of the container;

- the additive contains at least one of the following substances: uranium oxide U ₃ O ₈ , uranium oxide U ₃ O ₇ , plutonium oxide PuO ₂ , thorium oxide ThO ₂ , gadolinium oxide Gd ₂ O ₃ , pore forming substances, lubricants, Sintering dopants;

- For the manufacture of mixed uranium oxide-plutonium oxide (MOX) fuel pellets, the container is placed in a closed enclosure such as a glove box, and powders of uranium oxide and plutonium oxide and additives are placed in the container, and then used from outside the closed enclosure Agitation of the container in a controlled manner;

- adding a lubricating material to the granular material and mixing the granular material with the lubricating material so as to distribute the lubricating material on the particles of the granular material, before forming the raw pellets by pressing the granular material obtained by stirring in a container;

- Mixing, in the presence of moving bodies, the granular material mainly comprising uranium oxide _UO2 obtained by agitating the converted powder with plutonium oxide _PuO2 powder before forming raw material pellets for the manufacture of MOX fuel.

For a better understanding of the invention, several embodiments of the methods according to the invention and specific devices for their implementation will be described below.

An essential aspect of the process of the present invention is that it makes possible, in a single pressing and mixing operation, the starting material obtained directly from the _UF6 conversion process for The granular material from which raw pellets are made.

The starting material is uranium oxide powder mainly comprising UO ₂ obtained directly by a uranium hexafluoride UF ₆ conversion process and more specifically a dry conversion process. This powder obtained at the outlet of the conversion unit has a composition generally defined in the form UO _2+x , this powder mainly contains UO ₂ and small amounts of other oxides such as U ₃ O ₈ and U ₃ O ₇ , it is possible to treat these small amounts of oxides The amount is adjusted. The powder obtained at the outlet of the transformation unit contains crystallites with a size of 0.1 μm to 0.4 μm, which may be partly agglomerated in the form of more or less brittle, generally medium-sized agglomerates of 0.5 to 20 μm. The density of this powder is always less than 2 g/cm ³ or even less than 1.5 g/cm ³ , and usually less than 1 g/cm ³ and approximately 0.7 to 0.9 g/cm ³ . The flowability of this powder was determined in terms of grams per second according to the standard test through a 15 mm orifice in a conical container, the powder had zero flowability, the powder could not flow through the orifice in the standard test.

All densities indicated herein are bulk densities measured using standard tests (unless otherwise stated).

This starting material of low density, small particle size and zero flowability cannot be used in the process of making raw pellets without processing.

This starting material is also unsuitable for the manufacture of raw pellets without intermediate treatment, for example when it consists of wet-process obtained _UO2 oxide powder.

The method according to the present invention comprises only a single stage, applied in a container, of the above-mentioned starting powder into a granular material having a density above 1.7 g/cm ³ and good fluidity in the undensified state, wherein Place the starting powder and any additives in a container, as described below. The containers used generally have steel walls internally lined with a layer of synthetic organic material, such as polyurethane, in order to reduce or eliminate the risk of abrasion of the container walls and contamination of the product inside the container. The container wall generally has a shape obtained by rotating about an axis, for example cylindrical or annular. The container contains a moving compacted and mixed body which is preferably free within the container and/or can also be connected to the container in a movable manner.

The container is movably mounted on a stand and contains motion means that can apply agitation, so that the materials in the container, such as powders, are pressed and mixed with this motion, moving within the volume of the container with three-dimensional motion, i.e. their movement A vector has components along three axes in space that are not coplanar. Movement of the material within the container may be induced by agitation of the container alone or by both agitation of the container and lifting means within the container.

The moving parts inside the container are usually made of hard metals or alloys or ceramic materials. These moving parts preferably comprise sintered alumina or uranium oxide, thereby achieving the hardness required to prevent contamination of the uranium oxide powder with materials that could adversely affect the properties of the powder or neat pellets.

The moving compacted and mixed mass inside the container can have a variety of shapes, such as cylinders, spheres or cubes; these moving bodies can contain, for example, balls, rings, beads, cubes, cylinders with flat or semicircular ends, discs, or any other shape.

The capacity of the vessel can vary widely without affecting the conditions under which the process is carried out. The capacity of the container can vary from a few kilograms to hundreds of kilograms, or even up to several tons, and the capacity of the container corresponds to the maximum mass of the components (powder and moving objects) it can hold.

Depending on the capacity of the vessel, a number of parameters must be adjusted in order to obtain an optimum conversion yield of the uranium oxide powder into the granular material from which the feedstock pellets are made. These parameters are specifically the content of moving objects, which is defined as the ratio of the total volume of moving objects to the usable volume of the container; the powder filling rate, which is defined as the ratio of the total volume of powder placed in the container to the usable volume of the container, and Powder/moving object fill factor, defined as the ratio of the total volume of powder placed in a container to the interstitial volume of a moving object when the container is at rest.

Containers are usually filled in such a way that all moving objects are covered and gaps between moving objects are filled. Other fill conditions may also be used.

The vessel containing the movable body is preferably vibrated mounted on a stationary support and comprises a vibratory drive, usually comprising an unbalanced motor.

In a particular embodiment where it has been shown that dry converted _UO2 powder can be used to produce granular material well, a commercially available grinder manufactured by the company SWECO with the reference DM1 is used.

The grinder container has an annular wall and is mounted with its vertical axis of rotation on a fixed support via a vertical axis helical support spring. Install the vibration motor firmly on the container wall, and make its axis along the vertical axis of the container. The motor is unbalanced in weight so that when it is rotated, it will drive the container in a three-dimensional pendulum vibratory motion, with the container's axis rotating and oscillating simultaneously. The container contains a freely moving object, which may be eg spherical or cylindrical or more complex in shape, on which the powder to be treated is poured before the motor agitating the container is activated. Free-moving objects and powders are moved by the motion and vibration of the container, filling a large part of the internal volume of the container in three-dimensional trajectories. During the movement of the free-moving objects and the powder under the motion and vibration of the container, the powder is compressed between the moving objects and between the moving object and the container wall.

Unexpectedly, when uranium oxide powder produced by the _UF6 dry conversion process was put into the SWECODM1 mill, it was observed that the particle size of the powder continued to increase with time. SWECO equipment is described by its manufacturer as a vibratory mill that reduces the particle size of granular materials or powders down to the sub-micron scale of 0.5 μm. The dry converted powder has a density close to 0.8 g/cm ³ and contains crystallites of 0.1 μm to 0.4 μm which are to a greater or lesser extent inter-bonded in the form of agglomerates, in this known device The processing produces granules by compaction between moving bodies, the size of which becomes uniform over time and lies between 10 μm and 150 μm. The density of the powder continued to increase with the processing time in the container containing the moving objects until after about one to two hours a density value of about 2 g/cm ³ was reached. As mentioned above, the flowability of dry transformed powders is zero, and after a few minutes of vibration treatment in a container containing moving objects, the flowability of particles exceeds 10 g/s and can quickly reach up to 60 g/s and more Significantly higher values (measured using the equipment described above).

After one or two hours of treatment in a container containing moving bodies, and in some cases up to 150 minutes, the resulting granular material can be used to produce raw pellets by pressing, since its density, fluidity and compressibility are due to Shape of the resulting particles and their particle size.

Furthermore, the treatment in a vibrating vessel containing moving bodies can simultaneously produce an _intimate mixing of uranium oxide powder obtained by _the dry conversion process with additives such as _PuO2 , _ThO2 , _Gd2O3 and _Er2O3 or again Pore formers such as organic or inorganic substances that are easily destroyed during sintering, or lubricants such as zinc stearate or aluminum stearate or ethylene bisstearamide (bistearamide), or to adjust the crystal structure of the sintered pellets dopant. Any other additives that can modify the structure and composition of the pellets, such as those described above, may be added to the in-vessel mixture.

It is also possible to add to the mixture lubricants which also have a pore-forming effect and are used instead of conventional pore-forming agents, for example the product named AZB or aluminum oxalate or its derivatives.

The additives can be added in whole or in part to the oxide powder (comprising UO ₂ and U ₃ O ₈ ) either when the vessel is filled before starting the process or at specific times during the process.

A lubricant(s) may be added and mixed with the granules, which may or may not already be formed in the starting powder, in order to obtain a lubricating effect during the subsequent compression stage.

The method according to the invention can produce fuel mainly comprising uranium oxide _UO2 and other materials such as neutron absorbing materials such as gadolinia or erbium oxide or fissile fuel substances such as plutonium oxide or other fissionable products such as thorium oxide. These preparations are added to the container at the desired time in order to obtain a good mixing of these materials with the particles formed of uranium oxides produced by the powder obtained from UF ₆ conversion.

In the case of toxic and/or radioactive materials such as plutonium oxide, precautions well known to those skilled in the art must of course be taken. However, the use of the method according to the invention makes it possible to obtain, in a single operation, granular material usable for the production of raw pellets in a single container containing a moving compacted body, from which Preparation, addition of fillers and vibratory operation of the container can be controlled externally so that these operations can be performed conveniently and without danger to the operator.

In the case of a preferred embodiment, the kinematic pressing and mixing mass is completely free inside the container and forms part of the filling in the container. In this case, pressed objects such as beads or cylinders are first put into a container in a predetermined amount. Then the powders obtained directly from UF ₆ and the conversion process and different additives (if appropriate) are added. The container is then set into motion, preferably in an oscillating motion. The moving object is always kept in the container, and the processed material is discharged through the grid at the bottom of the container.

According to a preferred embodiment, the moving body inside the container is extremely hard aluminum oxide Al ₂ O ₃ . When carrying out the method according to the invention, the kinetic energy transferred to the moving bodies for compacting the powder is moderate, so that the energy used in the collisions between the moving bodies and between the moving parts and the walls of the container is low. The moving compacted and mixed body is thus subject to very limited damage and thus the powder is not contaminated by substances from the moving compressed body. Furthermore, adding small amounts of alumina to fuel pellets is not detrimental, and aluminum can even provide beneficial effects. Further measurements showed that aluminum contributed to no more than a few parts per million of this contamination.

When using sintered uranium oxide _UO2 moving bodies instead of alumina bodies, the risk of contamination by elements not present in uranium oxide powder is completely eliminated. In addition, the kinetic energy is increased due to the high density of the sintered uranium oxide body (theoretical density is 10.96 g/cm ³ ), thereby reducing the processing time. However, the addition of uranium in the form of sintered uranium oxide to the powder greatly reduces the sinterability of the mixture due to some degradation in the moving body and yields no other significant advantages.

Several embodiments will be described below for a better understanding of the invention.

Three embodiments of the present invention, respectively represented by Examples 1, 2 and 3, and a comparative example will be described below.

The characteristic stage of the process according to the invention consists in the treatment of the powder directly obtained from the dry conversion process in a container containing moving bodies in order to obtain a granular material which can be used for the production of raw pellets, commercially available under the name DM1 produced by the company SWECO This treatment is carried out in a commercially available vibrating mill.

In the case of the four examples, the annular vertical axis milling vessel contained a 20 kg moving body consisting of a sintered alumina cylinder approximately 1/2" (12.7 mm) in diameter and length. When processing the powder was poured On a moving pressed object, the object is completely free inside the container.Then start the unbalanced motor attached to the container to vibrate the container.

Example 1

Continuous preparation of several granular material fillers from uranium oxide _UO2 produced by the dry conversion process of _UF6 hexafluoride. To prepare the eight fillers in the first series, 10 kg of material in powder form containing 89 wt% uranium oxide UO ₂ , 6% UROX and 5% U ₃ O ₈ was placed in the container of the mill, to which was added 0.2% ethylene bis stearamide and 0.55 wt% ammonium oxalate, the mill vessel contained alumina moving bodies.

In order to prepare the eight fillers in the second series, 0.2 wt% of ethylene bis stearylamide and 0.47 wt% of ammonium oxalate were added to the same composition as the eight fillers in the first series, and they were placed in succession. into the container.

The article named UROX is uranium oxide U 3 O ₈ obtained from uranium oxyfluoride UO ₂ F ₂ in the transformation of UF ₆ to obtain uranium oxide, or uranium oxide _{U 3 from} UO ₂ oxide in a high-temperature furnace O ₈ .

_U3O8 oxides and UROX added to the mixture are recovered in _the fuel pellets during or after the production of the pellets.

The various fillers in the first series of fillers and the various fillers in the second series of fillers are agitated in the container, so that the fillers and moving objects move in all directions in space.

After a treatment time of about 120 minutes, a granular material mainly comprising uranium oxide was obtained in the vessel, which material had the following properties:

Average density (DNT) in the undensified state: about 2.2 g/cm ³ .

Average density (DT) in densified state: about 2.9 g/cm ³ .

Average fluidity: about 57g/s.

The particles in each series of eight fillers were homogenized in a mixer in order to give them uniform properties (in particular density and fluidity centered on the values mentioned above).

To homogenize the individual fillers of the two series, eight filler particles were placed in a rotating vessel mixer commonly used in nuclear fuel pellet manufacturing equipment and the vessel was rotated.

The mixer vessel is preferably biconical in shape. Such vessels, commonly referred to as double-cone mixers, are conventionally used in nuclear fuel fabrication facilities.

A double cone with a capacity of at least 80 kg is used, into which are placed eight fillers of each series of fillers, and the double cone is then rotated for about 5 minutes in order to obtain a homogeneous granular material having the above average properties.

It is quite obvious that in the industrial application of the present invention, when an agitation container with sufficient capacity (for example 80 kg) and with moving bodies is available, it is possible to agitate the starting powder mixture in a single operation in order to obtain a homogeneous granular material without the need for Further homogenization is performed in the bicone.

However, a double cone is used after homogenizing the filler or after agitating the powder in a large volume container to mix the lubricant with the granular material.

In the case of homogenizing the packing with a double cone, the lubricant is added to the homogenized packing in the double cone, and in the case of obtaining the whole granular material in a single operation, the granular material is transferred into the double cone and the lubricant is added.

For example, 0.25% by weight of ethylene bis stearamide is added to the particulate material. The lubricant was mixed with the particulate material by rotating the double-cone container for about 1 minute and 30 seconds.

The ethylene bisstearamide chosen as lubricant is preferably in the form of the commercial product CIREC from the company HOECHST, which has the particle size required to ensure optimum lubrication.

However, agitation in the presence of moving bodies causes interactions, and the presence of "hard" mixtures of substances tends to reduce or impair the lubricating effect of substances added as lubricants.

In order to obtain satisfactory lubricating conditions, it is therefore necessary to produce a "soft" mixture of particles and lubricant in a mixing device such as a double cone.

Increased compaction force and grinding reflect insufficient lubrication due to friction of the particles during compaction.

A homogenized and lubricated particulate material was obtained having a DNT of about 2.4 g/cm ³ and a DT of about 2.9 g/cm ³ . The fluidity of the particle mixture is about 80 g/s.

Two fillers of granular material obtained from the mixture differ only in the proportion of pore-forming material (ammonium oxalate) in the starting The material is compacted in order to obtain raw material pellets.

The raw pellets have good strength properties.

The density of the raw pellets obtained from the first filler (containing 0.55% porogen) was 6.3 g/cm ³ , while the density of the raw pellets obtained from the second filler (containing 0.47% porogen) was 5.8 g/cm ³ .

The desired sintered density (95%) can be achieved by adding porogens.

Properly disperse the lubricant into the granular material in both cases. No grinding was observed during pressing.

The stage of agitation of the powder mixture in the presence of moving bodies makes it possible to obtain granular material, the density of which in the undensified state and in the densified state is very significantly greater than that of the starting powder, which consists mainly of hexafluoro Uranium oxide UO ₂ obtained by the dry conversion process of uranium oxide UF ₆ .

The stage of mixing with the lubricant can slightly increase the density (at least in the undensified state) and significantly improve the flowability of the granular material.

Example 2

5 kg of uranium dioxide _UO2 powder is placed in a container containing a moving alumina object, this _UO2 powder is obtained directly from the _UF6 transformation process by dry method and cannot flow through a 15mm orifice, its density is 0.9g/cm ³ . No pore formers or lubricants were added to the _UO2 powder charge, and the treatment was started by shaking the container containing the charge.

Particulate material was removed after 10, 15, 30, 60 and 120 minutes of treatment, and treatment was terminated after 120 minutes.

The density and fluidity of the granular material were measured by sieving as described above and the results are shown in Table 2.

Table 2

test Filling method: 20kg alumina grinding media (11-13mm) No2 processing time minute 0 10 15 30 60 120 density DNT g/cm³ 0.9 1.3 1.3 1.5 1.8 2.1 DT g/cm³ 1.6 1.9 1.9 2.1 2.4 2.6 fluidity 15mm cone g/s 0 0 42 58 66 79

During the two hours of treatment, the density in the undensified state increased from 0.9 g/cm ³ to 2.1 g/cm ³ . After 15 minutes of treatment, the fluidity increased dramatically and reached a value of 79 g/s at the end of the treatment.

The granular material obtained from this treatment is compressed in the mold used to obtain the raw material pellets, as described above. For this operation, a lubricant is mixed with the granules in order to aid in the compaction of the granules in the form of raw material pellets. (If desired, a pore-forming material may be added to the pellet material prior to compaction in order to adjust the density of the sintered pellets to the desired value).

Example No3

table 3

test Filling method: 20kg alumina grinding media (11-13mm) No2 processing time minute 0 60 density DNT g/cm³ 1.1 2.1 DT g/cm³ 1.8 2.6 fluidity 15mm cone g/s 0 65

4 kg of uranium dioxide UO ₂ powder, obtained by a dry process, unable to flow through an orifice of 15 mm and having a density of 0.8 g/cm ³ , _is charged into a container containing a cylindrical pressed body, and then charged into 2 kg A uranium oxide mixture containing 25% gadolinium oxide (Gd ₂ O ₃ ) and 36 g of pore-forming material containing organic matter. The treatment was stopped after one hour, the powder comprising the mixture of uranium oxide and gadolinia had a density of 1.1 g/cm ³ at the start of the treatment and 2.1 g/cm ³ after one hour of treatment. The powder has a good flowability of 65 g/s.

To the powder was added 18 g of a lubricant comprising zinc stearate. The particulate material mixed with the lubricant is then directly compressed and formed into feed pellets which are then sintered using conventional techniques.

comparative example

The undensified uranium oxide _UO2 powder with a density of states of 0.85 g/cm ³ directly obtained by the dry transformation process is put into a knife grinder, such as the blades used for processing powders in transformation equipment according to the methods of the prior art grinder.

After treatment in the blade grinder, the density of the powder was unchanged or even slightly reduced to 0.8 g/cm ³ . The flowability of the powder was zero at the beginning and remained at zero when the processing in the blade grinder was completed.

The examples according to the present invention thus show that the method of the present invention, using an agitated vessel containing moving bodies and preferably free moving bodies, can significantly increase the density of powders obtained by the dry conversion process to close to or slightly higher than 2 g/ ^cm3 value. In addition, the treatment results in a granular material having excellent fluidity, which can be easily shaped into raw pellets using conventional methods.

The sintered feedstock pellets have the properties of fuel pellets produced according to prior art methods.

Furthermore, tests carried out have shown that additives can be added to uranium oxide powder obtained by the dry process before starting the treatment in the vessel, or during or at the end of the treatment in the vessel. When organic porogens are used, it is generally necessary to add more than 0.1 wt% porogen to the process vessel with uranium oxide, and in all cases greater than 0.01 wt%. Since each additive or other material has its own pore-forming effect, knowing the final density that needs to be obtained and the sintered density of the matrix without additives, the amount of additives and pore-forming materials that need to be added to obtain the required sintered density can be calculated. The method according to the invention comprises only one stage (or at most two stages, if the lubrication stage of the "soft" mixture is taken into account) to make uranium oxide powder from the UF6 conversion process which can be shaped into feedstock pellets The granular material, instead of the seven stages in the existing method, can greatly simplify the process and equipment used for nuclear fuel production.

The lubricant and particulate material can be mixed in devices other than double-cone mixers; such devices must provide a "soft mix" so as not to damage the lubricant.

The invention is applicable to the production of fuel pellets of widely varying composition.

In the production of MOX fuel pellets containing uranium oxide (mainly _UO2 ) and plutonium oxide _PuO2 , plutonium oxide can be added to the mixture prior to agitation in the presence of moving bodies, or added to a grinding vessel for agitation in the resulting granular material. The plutonium oxide in powder form may be mixed with the particulate material in powder form.

A number of (lubricating) substances may be added to the particulate material prior to forming the raw pellets, one or more lubricants selected from which may be used. Specifically, these lubricants may be, for example, ethylene bis stearamide or ADS (aluminum distearate).

Claims (16)

1. Method for producing nuclear _{fuel pellets by sintering a material comprising uranium dioxide UO2 obtained} from powder produced by the conversion process of uranium hexafluoride _UF6 , characterized in that it will be produced directly from hexafluoride The powder obtained by the fluoride UF ₆ transformation process is placed in a container containing a moving compaction and mixing body, and the container is agitated so that the powder moves along three non-coplanar axes in the internal volume of the container, so that the moving body The powder is pressed between the moving body and the container wall until a granular material with an undensified density of state of at least 1.7 g/ ^cm3 is formed, and the resulting granular material is agitated in the container to form raw fuel pellets for sintering grain. 2. A method according to claim 1, characterized in that the container is subjected to a three-dimensional vibratory movement. 3. A method according to any one of claims 1 and 2, characterized in that the powder placed in the container is obtained by a dry conversion process. 4. Process according to claim 1, characterized in that the powder has a density of less than 1 g/cm ³ , while the undensified state density of the granular material obtained by stirring in the container is about 2 g/cm ³ . 5. The method according to claim 1, characterized in that the powder directly obtained by the hexafluoride UF _{transformation} process has a density of ^less than 1 g/cm and zero fluidity determined by a standard test with a 15 mm orifice, and is in the container The granular material obtained by agitation in the container has a fluidity greater than 10 g/s after several minutes of agitation in the container. 6. The method according to claim 1, characterized in that the container containing the moving body and the powder obtained by the UF ₆ transformation process of hexafluoride is agitated for 1 to 600 minutes. 7. The method according to claim 1, characterized in that the moving body is made of one of the following materials: sintered alumina Al ₂ O ₃ , sintered uranium oxide, pure or doped sintered zirconia, tungsten carbide, steel , metal uranium or uranium/titanium alloy. 8. The method according to claim 1, characterized in that at least one additive comprising at least one porogen is added together with uranium dioxide _UO2 powder obtained directly by means of the hexafluoride _UF6 conversion process before the container is agitated Add to the container in a proportion of at least 0.01%. 9. The method according to claim 1, characterized in that at least one additive is added to the vessel together with the uranium dioxide _UO2 powder obtained directly by means of the hexafluoride _UF6 conversion process. 10. A method according to claim 9, characterized in that the additive is placed in the container prior to the treatment by agitating the container. 11. A method according to claim 9, characterized in that the additive is placed in the container during the treatment by agitating the container. 12. The method according to claim 9, characterized in that the additive comprises at least one of _the following substances _: uranium oxide _U3O8 , uranium oxide _U3O7 , plutonium oxide _PuO2 , thorium oxide _ThO2 , gadolinium oxide Gd ₂ O ₃ , pore forming substances, lubricants, sintering dopants. 13. The method according to claim 1 for the manufacture of mixed uranium oxide-plutonium oxide fuel pellets, characterized in that the container is placed in a closed enclosure, and powders of uranium oxide and plutonium oxide and additives are placed in the container, The container is then agitated by means of control from outside the enclosure; 14. The method of claim 13, wherein the enclosure is a glove box. 15. The method according to claim 1, characterized in that before the granular material obtained by stirring in the container is pressed to form the raw material pellets, a lubricating material is added to the granular material and a soft mixture of the granular material and the lubricating material is prepared so that the The lubricating material is distributed over the particles of the particulate material. 16. The method according to claim 1, characterized in that the granular material mainly comprising uranium oxide _UO2 obtained by agitating the conversion powder is mixed with oxidized Plutonium PuO ₂ powder mixed.